At the end of the Ancient Hospital Ruins you will have to fight the Icefire Wolf Ghost, who is a formidable enemy by any standards.
The Icefire Wolf Ghost will change it's type at the beginning of the battle then you must attack it with the opposing type of spell. It will also change it's type after each hit so you will need to alternate your attack, when he changes to water use flare and when he changes to fire use water. Keep using the attacks he is weak against and you will have beaten him in no time, however keep an eye on your characters health as he is capable of inflicting some heavy damage. Also if one of your characters is following the Assassin class you have a small chance of stealing a silver ring from him.
In the Ancient Hospital ruins you will come across two similar enemies they are
Flame Wolf Ghost Snow Wolf Ghost
These enemies can both do some serious damage to your party if given the chance however if you can pull them into a Monster Fight they will cancel each other out and become Scavenger Wolf Ghosts which are much easier to kill.
If you manage to stick with the Assassin class with at least one character until level 11 you will unlock the Negotiate Skill which will cause every shop to lower their prices by 50% when equipped.
Also you will unlock Long-Range Attack at Level 26 which will allow you to attack an enemy even if they are on the Back row.
At level 33 you will learn Double Strike which causes you to hit the enemy twice when you attack causing double damage, link this up with the Absorb HP and Absorb MP abilities from the Sword Master Class and charge attack from the Monk class and you make normal physical attacks one of the best attacks to use in the game.
Try to make sure that all of your characters use the Sword Master skill till at least level 17 as this will unlock Mow Down 2. This attack will allow you to attack the entire front row of enemies at once, in some battles this will allow you to destroy every enemy without them landing a single hit on your party.
If you are affected by any of the Drain spells (drain Strength, Endurance, life etc) then you can simply pray at the alter in one of the chapels and all negative effects will be removed from your character. However if you have the Shivering Isles Expansion pack installed you will not be able to use the Chapel of Dibella in Anvil (I think the Body in the Alter may have broken it).
At the end of the Lot Wilderness you will come up against the Flamboyant Dinoram (The Sheep Tribesman asks you to defend him from it). The Dinoram can be difficult to beat if you do not have the Flare spell so make sure you have it and at least 1 Black magic user in your party. If you do not have it go back for it at the beginning of the Lot Wilderness. As soon as the battle starts have your black magic user cast Flare at his Horn which will cause him to lower his head. Then he will be vulnerable to physical attacks or Magic sword attacks to his horn. When he raises his head again simply cast Flare at his horn again to make him lower it again. It may help if you have both Jiro and Kluke as Black magic users and Shu using physical attacks however you will not have the benefit of healing magics unless you have unlocked white magic lv1 with at least one character.
Near the beginning of the game (after you have crashed the mechat) when you get to the Lot Wilderness for the first time you can get the Flare Spell. When you first get into the Lot Wilderness you will see a large tree with a crack near the bottom. Go over to this tree and push it over then you can use it to get to the chest containing the Lv1 Flare spell which will prove very useful in an upcoming boss fight. Also just round the corner there is another tree that can be pushed over to reach a chest containing 300 gold.